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'The first airstrike killed one policeman. When other policemen came to help, they came under a second airstrike,' police said

A US air raid has killed eight Afghan policemen in the country's volatile south, officials said on Monday, in the first apparent "friendly fire" incident since American forces were granted greater powers to strike at insurgents.

The incident occurred on Sunday in the Tali area of Uruzgan province, where the Taliban recently attempted to overrun the capital city Tarin Kot in a major security breach.

"The first airstrike killed one policeman. When other policemen came to help, they came under a second air strike, killing seven of them," Rahimullah Khan, Uruzgan highway police commander, told AFP.

Mohammad Sediq, a policeman who survived the attack said their forces were "engaged in close fighting" with the Taliban when they were bombed.

The NATO command centre in Kabul confirmed US warplanes had conducted an air strike in the area, but said they targeted individuals posing a threat to Afghan forces.

"US forces conducted two airstrikes against individuals firing on our Afghan partners in Tarin Kot on 18 September," NATO spokesman Charles Cleveland said in a statement.

"We don't have any further information on who those individuals might have been or why they were attacking (Afghan) forces. US, coalition, and Afghan forces have the right to self-defence, and in this case were responding to an immediate threat."

NATO officially ended its combat mission in December 2014, but US forces got more power in June to strike at the insurgents as President Barack Obama vowed a more aggressive campaign.

The new authority gave the US-led NATO troops greater latitude to order airstrikes in support of Afghan troops.

Earlier this month, Afghan forces backed by US air strikes mounted an offensive to flush out Taliban insurgents encircling Tarin Kot.